U.S. patent application number 11/946211 was filed with the patent office on 2009-05-28 for method of operating a pyrolysis heater for reduced nox.
This patent application is currently assigned to AIR PRODUCTS AND CHEMICALS, INC.. Invention is credited to Reed Jacob Hendershot, Xianming Jimmy Li, Aleksandar Georgi Slavejkov.
Application Number | 20090136880 11/946211 |
Document ID | / |
Family ID | 40342204 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090136880 |
Kind Code |
A1 |
Hendershot; Reed Jacob ; et
al. |
May 28, 2009 |
Method Of Operating A Pyrolysis Heater For Reduced NOx
Abstract
A method of operating a pyrolysis heater for reduced emissions
of NOx and carbon monoxide. One or more wall burners, typically
premix burners, are operated with more excess oxidant gas than one
or more of the floor or hearth burners, which are typically
non-premix burners. The invention takes advantage of different NOx
emissions characteristics from different types of burners.
Inventors: |
Hendershot; Reed Jacob;
(Breinigsville, PA) ; Li; Xianming Jimmy;
(Orefield, PA) ; Slavejkov; Aleksandar Georgi;
(Allentown, PA) |
Correspondence
Address: |
AIR PRODUCTS AND CHEMICALS, INC.;PATENT DEPARTMENT
7201 HAMILTON BOULEVARD
ALLENTOWN
PA
181951501
US
|
Assignee: |
AIR PRODUCTS AND CHEMICALS,
INC.
Allentown
PA
|
Family ID: |
40342204 |
Appl. No.: |
11/946211 |
Filed: |
November 28, 2007 |
Current U.S.
Class: |
431/10 ; 431/12;
431/181 |
Current CPC
Class: |
F23C 6/047 20130101;
F23L 2900/07007 20130101; C10G 9/206 20130101; C10G 9/20 20130101;
Y02E 20/34 20130101; Y02E 20/344 20130101; C10G 9/14 20130101; F23C
2900/05081 20130101; F23C 5/08 20130101; F23C 2201/101
20130101 |
Class at
Publication: |
431/10 ; 431/12;
431/181 |
International
Class: |
F23D 14/32 20060101
F23D014/32; F23D 14/02 20060101 F23D014/02 |
Claims
1. A method of operating a pyrolysis heater comprising: introducing
a first fuel and a first oxidant gas into the pyrolysis heater
through a first wall burner positioned in a row of wall burners,
the first wall burner having a first equivalence ratio,
.phi..sub.1; and introducing at least one of the first fuel and a
second fuel and at least one of the first oxidant gas and a second
oxidant gas into the pyrolysis heater through a first floor burner
positioned in a row of floor burners, the first floor burner having
a second equivalence ratio, .phi..sub.2; wherein
.phi..sub.1<.phi..sub.2.ltoreq.1.0.
2. The method of claim 1 wherein
.phi..sub.1<0.95.times..phi..sub.2.
3. The method of claim 1 wherein .phi..sub.1<0.91 and
0.91<.phi..sub.2.
4. The method of claim 1 wherein the first wall burner is a premix
burner.
5. The method of claim 1 wherein the first floor burner is a
non-premix burner.
6. The method of claim 5 wherein the first wall burner is a premix
burner.
7. The method of claim 1 further comprising: introducing the first
fuel and the first oxidant gas into the pyrolysis heater through a
remaining set of wall burners positioned in the row of wall
burners, each of the remaining set of wall burners having a
respective wall burner equivalence ratio, wherein each respective
wall burner equivalence ratio is within 2% of the first equivalence
ratio; and introducing at least one of the first fuel and the
second fuel and at least one of the first oxidant gas and the
second oxidant gas into the pyrolysis heater through a remaining
set of floor burners positioned in the row of floor burners, each
of the remaining set of floor burners having a respective floor
burner equivalence ratio, wherein each respective floor burner
equivalence ratio is within 2% of the second equivalence ratio.
8. The method of claim 7 wherein the first wall burner and the
remaining set of wall burners are premix burners.
9. The method of claim 7 wherein the first floor burner and the
remaining set of floor burners are non-premix burners.
10. The method of claim 9 wherein the first wall burner and the
remaining set of wall burners are premix burners.
11. The method of claim 1 further comprising: introducing the first
fuel and the first oxidant into the pyrolysis heater through a
second wall burner positioned in the row of wall burners, the
second wall burner having a third equivalence ratio, .phi..sub.3,
the second wall burner located adjacent to the first wall burner
and spaced a first horizontal distance, d.sub.1, from the first
wall burner; and introducing at least one of the first fuel and the
second fuel and at least one of the first oxidant gas and the
second oxidant gas into the pyrolysis heater through a second floor
burner positioned in the row of floor burners, the second floor
burner having a fourth equivalence ratio, .phi..sub.4, the second
floor burner located adjacent to the first floor burner and spaced
at a second horizontal distance, d.sub.2, from the first floor
burner; wherein
1.4.times.d.sub.2.ltoreq.d.sub.1.ltoreq.2.1.times.d.sub.2.
12. The method of claim 11 wherein
.phi..sub.3<.phi..sub.4.ltoreq.1.0
13. The method of claim 12 wherein
.phi.3<0.95.times..phi..sub.4.
14. The method of claim 11 wherein .phi..sub.3<0.91 and
0.91<.phi..sub.4.
Description
BACKGROUND
[0001] The present invention relates to a heater for the pyrolysis
of hydrocarbons and particularly to a method of operating a
pyrolysis heater with reduced NOx emissions.
[0002] A pyrolysis heater may also be referred to as a pyrolysis
furnace. A pyrolysis heater is any device for the pyrolysis or
steam cracking of hydrocarbons.
[0003] The steam cracking or pyrolysis of hydrocarbons for the
production of olefins is almost exclusively carried out in tubular
coils placed in fired heaters. The pyrolysis process is considered
to be the heart of an olefin plant and has significant influence on
the economics of the overall plant.
[0004] The hydrocarbon feedstock may be any one of the wide variety
of typical cracking feedstocks such as methane, ethane, propane,
butane, mixtures of these gases, natural gas, naphthas, gas oils,
etc. The product stream contains a variety of components, the
concentration of which are dependent in part upon the feed
selected. In the conventional pyrolysis process, vaporized
feedstock is fed together with dilution steam to a tubular reactor
located within the fired heater. The quantity of dilution steam
required is dependent upon the feedstock selected; lighter
feedstocks such as ethane require lower steam (0.2 lb./lb. feed),
while heavier feedstocks such as naphtha and gas oils generally
require steam/feed ratios of 0.5 to 1.0. The dilution steam has the
dual function of lowering the partial pressure of the hydrocarbon
and reducing the carbon laydown rate in the pyrolysis coils.
[0005] In a typical pyrolysis process, the steam/feed mixture is
preheated to a temperature just below the onset of the cracking
reaction, typically 650.degree. C. This preheat occurs in the
convection section of the heater. The mixture then passes to the
radiant section where the pyrolysis reactions occur. Generally the
residence time in the pyrolysis coil is in the range of 0.2 to 0.4
seconds and outlet temperatures for the reaction are on the order
of 700.degree. to 900.degree. C. The reactions that result in the
transformation of saturated hydrocarbons to olefins are highly
endothermic thus requiring high levels of heat input. This heat
input must occur at the elevated reaction temperatures. It is
generally recognized in the industry that for most feedstocks, and
especially for heavier feedstocks such as naphtha, shorter
residence times will lead to higher selectivity to ethylene and
propylene since secondary degradation reactions will be reduced.
Further it is recognized that the lower the partial pressure of the
hydrocarbon within the reaction environment, the higher the
selectivity.
[0006] The flue gas temperatures in the radiant section of the
fired heater are typically above 1,100.degree. C. In a conventional
design, approximately 32 to 40% of the heat fired as fuel into the
heater is transferred into the coils in the radiant section. The
balance of the heat is recovered in the convection section either
as feed preheat or as steam generation. Given the limitation of
small tube volume to achieve short residence times and the high
temperatures of the process, heat transfer into the reaction tube
is difficult. High heat fluxes are used and the operating tube
metal temperatures are close to the mechanical limits for even
exotic metallurgies. In most cases, tube metal temperatures limit
the extent to which residence time can be reduced as a result of a
combination of higher process temperatures required at the coil
outlet and the reduced tube length (hence tube surface area) which
results in higher flux and thus higher tube metal temperatures. The
exotic metal reaction tubes located in the radiant section of the
cracking heater represent a substantial portion of the cost of the
heater so it is important that they be utilized fully. Utilization
is defined as operating at as high and as uniform a heat flux and
metal temperature as possible consistent with the design objectives
of the heater. This will minimize the number and length of the
tubes and the resulting total metal required for a given pyrolysis
capacity.
[0007] In the majority of cracking furnaces, the heat is supplied
by floor burners, also called hearth burners, that are installed in
the floor of the firebox and fire vertically upward along the
walls. Because of the characteristic flame shape from these
burners, an uneven heat flux profile is created. The typical
profile shows a peak flux near the center elevation of the firebox,
with the top and bottom portions of the firebox remaining
relatively cold. In select heaters, radiant wall burners are
installed in the top part of the sidewalls to equalize the heat
flux profile in the top portion of the heater. Improving the heat
flux profile is complicated by NOx emission considerations.
[0008] Nitrogen oxides (NOx) are produced in essentially all
combustion processes using air as the oxidant gas. NOx is produced
primarily as nitric oxide (NO) in the hottest regions of the
combustion zone. Some nitrogen dioxide (NO.sub.2) is also formed,
but its concentration is generally a small percentage of the total
NOx.
[0009] Nitrogen oxides are among the primary air pollutants emitted
from combustion processes. NOx emissions have been identified as
contributing to the degradation of the environment, particularly
degradation of air quality, formation of smog (poor visibility) and
acid rain. As a result, air quality standards are being imposed by
various governmental agencies, which limit the amount of NOx gases
that may be emitted into the atmosphere.
[0010] In addition, there is an inverse relationship between NOx
and CO formation which further complicates emissions control.
Combustion processes do not perfectly bring together the three T's
(time, temperature, and turbulence) to achieve complete combustion,
and some amount of CO generation is inevitable. Generally speaking,
the higher the peak combustion temperature, the lower the CO
generation. Unfortunately, the trend is just the reverse for NOx
generation; the higher the combustion temperature, the greater the
NOx generation. Therefore, emission control for industrial
combustion sources must compromise between NOx and CO control.
BRIEF SUMMARY
[0011] The present invention relates to a method of operating a
pyrolysis heater.
[0012] The method comprises introducing a first fuel and a first
oxidant gas into the pyrolysis heater through a first wall burner
positioned in a row of wall burners, the first wall burner having a
first equivalence ratio, .phi..sub.1; and introducing at least one
of the first fuel and a second fuel and at least one of the first
oxidant gas and a second oxidant gas into the pyrolysis heater
through a first floor burner positioned in a row of floor burners,
the first floor burner having a second equivalence ratio,
.phi..sub.2; wherein .phi..sub.1<.phi..sub.2.ltoreq.1.0.
[0013] The method may comprise one or more of the following
characteristics, taken alone or in any possible technical
combination.
[0014] The first equivalence ratio, .phi..sub.1, may be less than
95% of the second equivalence ratio, .phi..sub.2. The first
equivalence ratio, .phi..sub.1, may be less than 0.91 and the
second equivalence ratio, .phi..sub.2, may be greater than
0.91.
[0015] The first wall burner may be a premix burner.
[0016] The first floor burner may be a non-premix burner.
[0017] The method may further comprise introducing the first fuel
and the first oxidant gas into the pyrolysis heater through a
remaining set of wall burners positioned in the row of wall
burners, each of the remaining set of wall burners having a
respective wall burner equivalence ratio, wherein each respective
wall burner equivalence ratio is within 2% of the first equivalence
ratio; and introducing at least one of the first fuel and the
second fuel and at least one of the first oxidant gas and the
second oxidant gas into the pyrolysis heater through a remaining
set of floor burners positioned in the row of floor burners, each
of the remaining set of floor burners having a respective floor
burner equivalence ratio, wherein each respective floor burner
equivalence ratio is within 2% of the second equivalence ratio.
[0018] The remaining set of wall burners may be premix burners.
[0019] The remaining set of floor burners may be non-premix
burners.
[0020] The method may further comprise introducing the first fuel
and the first oxidant into the pyrolysis heater through a second
wall burner positioned in the row of wall burners, the second wall
burner having a third equivalence ratio, .phi..sub.3, the second
wall burner located adjacent to the first wall burner and spaced a
first horizontal distance, d.sub.1, from the first wall burner; and
introducing at least one of the first fuel and the second fuel and
at least one of the first oxidant gas and the second oxidant gas
into the pyrolysis heater through a second floor burner positioned
in the row of floor burners, the second floor burner having a
fourth equivalence ratio, .phi..sub.4, the second floor burner
located adjacent to the first floor burner and spaced at a second
horizontal distance, d.sub.2, from the first floor burner, wherein
1.4.times.d.sub.2.ltoreq.d.sub.1.ltoreq.2.1.times.d.sub.2.
[0021] The third equivalence ratio, .phi..sub.3, may be less than
the fourth equivalence ratio, .phi..sub.4, and the fourth
equivalence ratio, .phi..sub.4, may be less than or equal to 1.
[0022] The third equivalence ratio, .phi..sub.3, may be less than
95% of the fourth equivalence ratio, .phi..sub.4.
[0023] The third equivalence ratio, .phi..sub.3, may be less than
0.91 and the fourth equivalence ratio, .phi..sub.4, may be greater
than 0.91.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0024] FIG. 1 is a perspective cutaway view of the lower portion of
a part of a pyrolysis heater.
[0025] FIG. 2 shows generalized plots of NOx emissions as a
function of equivalence ratio for a premix burner and a non-premix
burner.
DETAILED DESCRIPTION
[0026] The indefinite articles "a" and "an" as used herein mean one
or more when applied to any feature in embodiments of the present
invention described in the specification and claims. The use of "a"
and "an" does not limit the meaning to a single feature unless such
a limit is specifically stated. The definite article "the"
preceding singular or plural nouns or noun phrases denotes a
particular specified feature or particular specified features and
may have a singular or plural connotation depending upon the
context in which it is used. The adjective "any" means one, some,
or all indiscriminately of whatever quantity.
[0027] For the purposes of simplicity and clarity, detailed
descriptions of well-known devices, circuits, and methods are
omitted so as not to obscure the description of the present
invention with unnecessary detail.
[0028] The present invention relates to a method of operating a
pyrolysis heater for reduced NOx emissions. The method may be
applied to a conventional pyrolysis heater having floor burners and
wall burners.
[0029] FIG. 1 illustrates a perspective cutaway view of a portion
of the firebox 1 of a pyrolysis heater. The firebox 1 comprises
walls 30 and a floor (also called a hearth) 40. Mounted on the
floor 40 are vertically firing floor burners 10 which are directed
upward along the walls and are supplied with oxidant gas and fuel.
Floor burners 10 may be selected from any of the commercially
available burners used in pyrolysis heaters. Floor burners 10 may
be non-premix burners, meaning that fuel and oxidant gas are
separately introduced into the pyrolysis heater. Floor burners 10
are shown having oxidant gas nozzle 12 and fuel nozzle 14 for
separately introducing oxidant gas and fuel, respectively.
[0030] FIG. 1 also shows wall burners 20 mounted in or on the walls
30 of the pyrolysis heater. Wall burners 20 may be so-called premix
burners, where fuel and oxidant gas are mixed together prior to
being introduced into the burner or within the burner itself.
Suitable wall burners for pyrolysis heaters are known in the
art.
[0031] The method comprises introducing a first fuel and a first
oxidant gas into the pyrolysis heater through a first wall burner
20. The first wall burner has a first equivalence ratio,
.phi..sub.1, meaning that the first fuel and the first oxidant gas
are introduced with flow rates that provide the first equivalence
ratio, .phi..sub.1. The first wall burner 20 is positioned in a
horizontal row of wall burners. FIG. 1 shows 3 rows of wall burners
20 on the wall.
[0032] The method comprises introducing at least one of the first
fuel and a second fuel and at least one of the first oxidant gas
and a second oxidant gas into the pyrolysis heater through a first
floor burner 10. The first floor burner has a second equivalence
ratio, .phi..sub.2, meaning that the at least one of the first fuel
and the second fuel and the at least one of the first oxidant gas
and the second oxidant gas are introduced with flow rates that
provide the second equivalence ratio, .phi..sub.2. The first floor
burner 10 is positioned in a row of floor burners. FIG. 1 shows 2
rows of floor burners on the floor. The floor burners 10 may use
the same fuel (i.e. the first fuel) or a different fuel (i.e. the
second fuel) than the wall burners 20. The floor burners 10 may use
the same oxidant gas (i.e. the first oxidant gas) or a different
oxidant gas (i.e. the second oxidant gas) than the wall burners 20.
The at least one of the first fuel and the second fuel and the at
least one of the first oxidant gas and the second oxidant gas may
be introduced separately (i.e. non-premixed) into the pyrolysis
heater through the first floor burner 10.
[0033] According to the method
.phi..sub.1<.phi..sub.2.ltoreq.1.0. The first equivalence ratio,
.phi..sub.1, is less than the second equivalence ratio,
.phi..sub.2, and the second equivalence ratio, .phi..sub.2, is less
than or equal to 1. The first equivalence ratio may be less than
95% of the second equivalence ratio. The first equivalence ratio
may be less than 0.91 and the second equivalence ratio may be
greater than 0.91. An equivalence ratio of 0.91 corresponds to 10%
excess oxidant gas.
[0034] The "equivalence ratio" is a term used widely in combustion
research. The equivalence ratio is defined as the fuel:oxidant gas
ratio divided by the fuel:oxidant gas ratio corresponding to
complete combustion. The latter ratio (fuel:oxidant gas ratio
corresponding to complete combustion) is often referred to as the
stoichiometric fuel:oxidant gas ratio. An equivalence ratio of 1
means that fuel and oxidant gas are provided in the theoretically
correct or stoichiometric amount; an equivalence ratio of 1.0 is
the same as 0% excess oxidant gas or "on-ratio". An equivalence
ratio greater than 1 is fuel rich and an equivalence ratio less
than 1 is fuel lean.
[0035] Since .phi..sub.1<.phi..sub.2.ltoreq.1.0, it means that
both the first wall burner and the first floor burner are operated
in fuel lean mode, not substoichiometrically (i.e. not fuel rich).
By maintaining fuel lean combustion for both the first wall burner
and the first floor burner, CO emissions may be suppressed.
[0036] The first fuel and the second fuel may be selected from
natural gas, refinery fuel gas or any other fuel known in the art
for pyrolysis heaters.
[0037] The first oxidant gas and the second oxidant gas may be
selected from air, oxygen-enriched air, oxygen-depleted air,
industrial grade oxygen, or low-grade oxygen. The first oxidant gas
may be at ambient temperature or may be preheated to a higher
temperature. Air generally has an oxygen concentration of about
20.9 volume % oxygen, typically rounded off to 21 volume % oxygen.
As used herein, oxygen-enriched air has an oxygen concentration
greater than air up to and including 30 volume % oxygen;
oxygen-depleted air has an oxygen concentration less than air down
to 10 volume % oxygen (e.g. turbine exhaust); industrial grade
oxygen has an oxygen concentration greater than 85 volume % up to
100 volume %; and low-grade oxygen has an oxygen concentration
greater than 30 volume % up to and including 85 volume %.
[0038] The first wall burner may include fuel staging and/or
oxidant gas staging. As defined herein, the equivalence ratio is
calculated using the fuel flow rate and the oxidant gas flow rate
for the primary burner nozzle(s) and any associated fuel staging
lances and/or associated oxidant gas staging lances. A primary
burner nozzle is any nozzle that provides a flame anchoring point
wherein the ignition and continuous combustion of fuel with oxidant
gas are assured. A fuel staging lance is associated with the
closest primary burner nozzle. In case a fuel staging lance is
positioned midway between the primary burner nozzles of two
burners, half of the fuel is included with one of the burners and
half to the other burner. Similarly, an oxidant staging lance is
associated with the closest primary burner nozzle. In case an
oxidant gas staging lance is positioned midway between the primary
burner nozzles of two burners, half of the oxidant gas is included
in the equivalence ratio calculation with one of the burners and
half to the other burner.
[0039] The first floor burner may include fuel staging and/or
oxidant gas staging. The equivalence ratio for the floor burner is
calculated in the same way as described above for the first wall
burner.
[0040] The first wall burner may be a premix burner. All of the
wall burners may be premix burners. A premix burner is a burner
where fuel and oxidant gas are mixed prior to entering the heater
or furnace, and thereby imparts a functional characteristic to the
process. The fuel and oxidant gas may be mixed prior to entering
the burner or within the burner itself. As defined herein, a burner
is a premix burner if at least 50% of the fuel flow to the burner
and at least 50% of the oxidant gas flow to the burner is mixed
prior to introducing the fuel and the oxidant gas into the
pyrolysis heater through the burner. The balance of the fuel and/or
oxidant gas may be introduced through associated fuel lances or
associated oxidant gas lances.
[0041] The method may further comprise introducing the first fuel
and the first oxidant gas through the first wall burner where the
first wall burner is a premix burner. Alternatively stated, the
method may further comprise mixing at least 50% of the first fuel
flow to the first wall burner and at least 50% of the first oxidant
gas flow to the first wall burner prior to introducing the first
fuel and the first oxidant gas into the pyrolysis heater through
the first wall burner 20. The first fuel and the first oxidant gas
may be mixed prior to introducing into the burner or mixed within
the burner itself.
[0042] The first floor burner 10 may be a non-premix burner.
Non-premix burners are also referred to as diffusion flame-type
burners. A non-premix burner is a burner where fuel and oxidant gas
are introduced separately (i.e. through separate nozzles) into the
heater or furnace without prior mixing, and thereby imparts a
functional characteristic to the process. As defined herein, a
non-premix burner is any burner that is not a premix burner.
[0043] The at least one of the first fuel and the second fuel and
the at least one of the first oxidant gas and the second oxidant
gas may be introduced into the pyrolysis heater through the first
floor burner 10 separately, i.e. without mixing prior to
introducing the fuel and oxidant into the pyrolysis heater.
[0044] The effect of applying the method may be observed with the
help of FIG. 2. FIG. 2 shows generalized plots of relative NOx
emissions as a function of equivalence ratio for a premix burner
and a non-premix burner. As shown in the plots, the NOx emissions
from premixed burners and non-premixed burners vary depending on
the equivalence ratio. Generally, premixed burners have a higher
maximum NOx emissions than non-premixed burners, and the maximum
occurs near an equivalence ratio of 1 for premix burners. The
maximum NOx emissions for a non-premixed burner are generated at
equivalence ratios less than 1 (fuel lean).
[0045] It is conventional to operate all of the burners in a
pyrolysis heater at the same equivalence ratio, typically about
0.91, which corresponds to about 10% excess oxidant gas.
[0046] The inventors have discovered that overall pyrolysis heater
NOx emissions may be reduced by operating the wall burners and the
floor burners with different equivalence ratios, as per the method
disclosed herein.
[0047] As illustrated in FIG. 2, the equivalence ratio for the
floor burners (non-premixed) is increased closer to stoichiometric
conditions, but not substoichiometric. At the same time, the
equivalence ratio for the wall burners (premixed) is decreased. The
overall equivalence ratio for the pyrolysis heater may still be
about 0.91. Because the firing rate of the floor burners is higher
than the firing rate for the wall burners, the relative change in
the equivalence ratio for the wall burners is greater than for the
floor burners. For example, for a pyrolysis heater having a row of
floor burners along a wall and two rows of wall burners above, the
percent firing rate to the floor burner may be about 80%, while the
firing rate to the wall burners may be about 10% each. As depicted
in FIG. 2, this has a synergistic effect for the reduction of NOx
emissions.
[0048] As shown in FIG. 1, the wall burners 20 may have the same
horizontal spacing or a different horizontal spacing than the floor
burners 10. In FIG. 1, the lowest row of wall burners are shown to
have a spacing 2 times the spacing of the floor burners. The wall
burners are shown to align vertically with the floor burners.
Alternatively, some or all of the wall burners may be staggered
with respect to the floor burners.
[0049] The inventors have found that by increasing the spacing of
at least the lowest row of wall burners, flame interaction can be
decreased and thereby NOx emissions are also decreased. The lowest
row refers to the height in the pyrolysis heater. The lowest row of
wall burners may be spaced farther apart than the floor burners,
preferably 1.4 to 2.1 times the spacing of the floor burners.
[0050] The method may further comprise introducing the first fuel
and the first oxidant into the pyrolysis heater through a second
wall burner 20, and introducing at least one of the first fuel and
the second fuel and at least one of the first oxidant gas and the
second oxidant gas into the pyrolysis heater through a second floor
burner 10. The second wall burner 20 has a third equivalence ratio,
.phi..sub.3, meaning that the first fuel and the first oxidant gas
are introduced through the second wall burner with flow rates that
provide the third equivalence ratio, .phi..sub.3. The second floor
burner 10 has a fourth equivalence ratio, .phi..sub.4, meaning that
the at least one of the first fuel and the second fuel and the at
least one of the first oxidant gas and the second oxidant gas are
introduced through the second floor burner with flow rates that
provide the fourth equivalence ratio, .phi..sub.4. The second wall
burner 20 is positioned in the horizontal row of wall burners. The
second wall burner is located adjacent to the first wall burner and
is spaced a first horizontal distance, d.sub.1, from the first wall
burner. The second floor burner is located adjacent to the first
floor burner and is spaced at a second horizontal distance,
d.sub.2, from the first floor burner, where
1.4.times.d.sub.2.ltoreq.d.sub.1.ltoreq.2.1.times.d.sub.2.
[0051] As defined herein, the burner located adjacent to the first
wall burner is the adjacent burner having a non-zero firing rate.
In case a burner has no fuel firing rate, it is ignored for the
purpose of determining an adjacent burner.
[0052] According to this aspect of the method
.phi..sub.3<.phi..sub.4.ltoreq.1.0. The third equivalence ratio,
.phi..sub.3, is less than the fourth equivalence ratio,
.phi..sub.4, and the fourth equivalence ratio, .phi..sub.4, is less
than or equal to 1. The third equivalence ratio may be less than
95% of the fourth equivalence ratio. The third equivalence ratio
may be less than 0.91 and the fourth equivalence ratio may be
greater than 0.91.
[0053] All of the wall burners positioned in the row of wall
burners may be operated with substantially the same equivalence
ratio. Substantially the same equivalence ratio is defined herein
to mean within 2% of the value of the equivalence ratio for any one
of the burners in the row of wall burners. Consequently, the method
may further comprise introducing the first fuel and the first
oxidant gas into the pyrolysis heater through a remaining set of
wall burners positioned in the row of wall burners, each of the
remaining set of wall burners having a respective wall burner
equivalence ratio, wherein each respective wall burner equivalence
ratio is within 2% of the first equivalence ratio. The remaining
set of wall burners positioned in the row of wall burners are the
other wall burners in the row of wall burners, which when combined
with the first wall burner make up the entire row of wall burners.
Each of the wall burners has its respective equivalence ratio. The
equivalence ratio of each of the wall burners is within 2% of the
first equivalence ratio.
[0054] All of the floor burners positioned in the row of floor
burners may be operated with substantially the same equivalence
ratio. Substantially the same equivalence ratio has the same
meaning as stated above for the wall burners, but applied instead
to the floor burners. Consequently, the method may further comprise
introducing at least one of the first fuel and the second fuel and
at least one of the first oxidant gas and the second oxidant gas
into the pyrolysis heater through a remaining set of floor burners
positioned in the row of floor burners, each of the remaining set
of floor burners having a respective floor burner equivalence
ratio, wherein each respective floor burner equivalence ratio is
within 2% of the second equivalence ratio. The remaining set of
floor burners positioned in the row of floor burners are the other
floor burners in the row of floor burners, which when combined with
the first floor burner make up the entire row of floor burners.
Each of the floor burners has its respective equivalence ratio. The
equivalence ratio of each of the floor burners is within 2% of the
second equivalence ratio.
EXAMPLE
[0055] The method was applied to a pyrolysis heater. Initially the
pyrolysis heater operated with an equivalence ratio of about 0.91,
representing the base case conditions. The oxygen concentration in
the stack gases on a dry basis was about 2.7 volume %. Air to the
floor burners was reduced, thereby providing a higher equivalence
ratio and air to the wall burners was increased, thereby providing
a lower equivalence ratio. The oxygen concentration in the stack
gases on a dry basis was about 2.2 volume %. The NOx concentration
in the stack gases was reduced about 17% when only a 6% reduction
in NOx concentration would be expected based on a change in oxygen
concentration in the stack alone.
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